A non-contact type torque sensor that detects steering torque acting on a steering shaft by magnetic force is known as a torque sensor provided in a steering apparatus of a vehicle.
JP2009-244205A discloses a torque sensor including a magnetism generation portion fixed to an input shaft, a rotating magnetic circuit portion fixed to an output shaft, a fixed magnetic circuit portion fixed to a housing, and a magnetism sensor that detects a magnetic flux density guided to the fixed magnetic circuit portion.
When torque acts on a torsion bar that connects the input shaft to the output shaft so that the torsion bar undergoes torsional deformation, a relative position in a rotation direction between the magnetism generation portion and the rotating magnetic circuit portion varies. Accordingly, the magnetic flux density guided to the fixed magnetic circuit portion from the magnetism generation portion through the rotating magnetic circuit portion also varies. The magnetism sensor outputs a signal corresponding to the magnetic flux density. The torque acting on the torsion bar is detected on the basis of the signal output from the magnetism sensor.
SUMMARY OF INVENTION
Referring to FIG. 23, a path of magnetic flux accompanying torsional deformation of a torsion bar will be described. As shown by straight line arrows in FIG. 23, a magnetic flux path between a rotating magnetic circuit portion and a fixed magnetic circuit portion extends from an N pole of a permanent magnet 91 of a magnetism generation portion toward an S pole of the permanent magnet 91 via a first soft magnetic ring 92 of the rotating magnetic circuit portion, a first magnetic flux collecting ring (not shown) of the fixed magnetic circuit portion, a first magnetic flux collecting yoke 93, a second magnetic flux collecting yoke 94, and a second magnetic flux collecting ring (not shown), and a second soft magnetic ring 95 of the rotating magnetic circuit portion. A magnetism sensor 96 is disposed between the first magnetic flux collecting yoke 93 and the second magnetic flux collecting yoke 94.
Here, as shown by curved line arrows in FIG. 23, flux leakage exists in the rotating magnetic circuit portion between the first soft magnetic ring 92 and the second soft magnetic ring 95. When the magnetism sensor 96 is disposed close to the input and output shafts in order to reduce the size of the apparatus, the magnetism sensor 96 is exposed to the flux leakage in the rotating magnetic circuit portion so as to be affected by the flux leakage, and as a result, the torque sensor generates a detection error.
The present invention has been designed in consideration of the problem described above, and an object thereof is to improve a detection precision of a torque sensor.
According to one aspect of present invention, a torque sensor that detects torque acting on a torsion bar that connects a first shaft and a second shaft, which are supported in a housing to be free to rotate is provided. The torque sensor includes a magnetism generation portion that rotates together with the first shaft, a rotating magnetic circuit portion that rotates together with the second shaft, a fixed magnetic circuit portion fixed to the housing, a magnetism detector that detects a magnetic flux density guided from the magnetism generation portion to the fixed magnetic circuit portion through the rotating magnetic circuit portion in accordance with torsional deformation of the torsion bar, and a shield disposed between the rotating magnetic circuit portion and the magnetism detector in order to shield the magnetism detector magnetically.